1. Field of the Invention
The present invention relates to a coating system for the generation of protective surface alloys for high temperature metal alloy products and, more particularly, relates to the provision of a metal alloy coating on the internal wall surfaces of high-temperature stainless steel tubes to produce a coating that provides corrosion resistance and reduces the formation of catalytic coking in hydrocarbon processing such as in olefin production and in direct reduction of ores.
2. Description of the Related Art
Stainless steels are a group of alloys based on iron, nickel and chromium as the major constituents, with additives that can include carbon, tungsten, niobium, titanium, molybdenum, manganese, and silicon to achieve specific structures and properties. The major types are known as martensitic, ferritic, duplex and austenitic steels. Austenitic stainless steel generally is used where both high strength and high corrosion resistance is required. One group of such steels is known collectively as high temperature alloys (HTAs) and is used in industrial processes that operate at elevated temperatures generally above 650xc2x0 C. and extending to the temperature limits of ferrous metallurgy at about 1150xc2x0 C. The major austenitic alloys used have a composition of iron, nickel or chromium in the range of 18 to 41 wt % chromium, 18 to 48 wt % nickel, balance iron and other alloying additives. Typically, high chromium stainless steels have about 31 to 38 wt % chromium and low chromium stainless steels have about 20 to 25 wt % chromium.
The bulk composition of HTAs is engineered towards physical properties such as creep resistance and strength, and chemical properties of the surface such as corrosion resistance. Corrosion takes many forms depending on the operating environment and includes carburization, oxidation and sulfidation. Protection of the bulk alloy is often provided by the surface being enriched in chromium oxide (chromia) and aluminum oxide (alumina).
These two metal oxides, or a mixture thereof, are mainly used to protect alloys at high temperatures. The compositions of stainless steels for high temperature use are tailored to provide a balance between good mechanical properties and good resistance to oxidation and corrosion. Alloy compositions which can provide an alumina scale are favoured when good high temperature oxidation resistance is required, whereas compositions capable of forming a chromia scale are selected for resistance to hot corrosive conditions. Unfortunately, the addition of high levels of aluminum to the bulk alloy is not compatible with retaining good mechanical properties. Therefore applying a coating containing aluminum onto the bulk alloy is a good way to provide the desired alumina surface oxide while maintaining desired mechanical properties.
One of the most severe industrial processes from a materials perspective is the manufacture of olefins such as ethylene by hydrocarbon steam pyrolysis (cracking). Hydrocarbon feedstock such as ethane, propane, butane or naphtha is mixed with steam and passed through a furnace coil made from welded tubes and fittings. The coil is heated on the outerwall and the heat is conducted to the innerwall surface leading to the pyrolysis of the hydrocarbon feed to produce the desired product mix at temperatures in the range of 850 to 1150xc2x0 C. An undesirable side effect of the process is the buildup of coke (carbon) on the innerwall surface of the coil. There are two major types of coke: catalytic coke (or filamentous coke) that grows in long threads when promoted by a catalyst such as nickel or iron, and amorphous coke that forms in the gas phase and plated out from the gas stream. In light feedstock cracking, catalytic coke can account for 80 to 90% of the deposit and provides a large surface area for collecting amorphous coke.
The coke builds up and constricts flow in the tubes and acts as a thermal insulator, requiring a continuous increase in the tube outer wall temperature to maintain throughput. A point is reached when the coke buildup is so severe that either the pressure drop reaches unacceptable levels or the tube skin temperature cannot be raised any further and the furnace coil is then taken offline to remove the coke by burning it off (decoking). The decoking operation typically lasts for 24 to 96 hours and is necessary once every 10 to 180 days. During a decoke period, there is no marketable production which represents a major economic loss. Additionally, the decoke process degrades tubes at an accelerated rate, leading to a shortened lifetime. In addition to inefficiencies introduced to the operation, the formation of coke also leads to accelerated carburization, other forms of corrosion, and erosion of the tube inner wall. The carburization results from the diffusion of carbon into the steel forming brittle carbide phases. This process leads to volume expansion and the embrittlement results in loss of strength and possible crack initiation. With increasing carburization, the alloy""s ability of providing some coking resistance through the formation of a chromium based scale deteriorates. At normal operating temperatures, half of the wall thickness of some steel tube alloys can be carburized in as little as two years of service. Typical tube lifetimes range from 3 to 6 years.
It has been demonstrated that aluminized steels, silica coated steels, and steel surfaces enriched in manganese oxides or chromium oxides are beneficial in reducing catalytic coke formation. Alonizing(trademark), or aluminizing, involves the diffusion of aluminum into the alloy surface by pack cementation, a chemical vapour deposition technique. The coating is functional to form a NiAl type compound and provides an alumina scale which is effective in reducing catalytic coke formation and protecting from oxidation and other forms of corrosion. The coating is not stable at temperatures such as those used in ethylene furnaces, and also is brittle, exhibiting a tendency to spall or diffuse into the base alloy matrix. Generally, pack cementation is limited to the deposition of one or two elements, the co-deposition of multiple elements being extremely difficult. Commercially , it is generally limited to the deposition of only a few elements, mainly aluminum. Some work has been carried out on the codeposition of two elements, for example chromium and silicon. Another approach to the application of aluminum diffusion coatings to an alloy substrate is disclosed in U.S. Pat. No. 5,403,629 issued to P. Adam et al. This patent details a process for the vapour deposition of a metallic interlayer on the surface of a metal component, for example by sputtering. An aluminum diffusion coating is thereafter deposited on the interlayer.
Alternative diffusion coatings have also been explored. In an article in xe2x80x9cProcessing and Propertiesxe2x80x9d entitled xe2x80x9cThe Effect of Time at Temperature on Silicon-Titanium Diffusion Coating on IN738 Base Alloyxe2x80x9d by M. C. Meelu and M. H. Lorretto, there is disclosed the evaluation of a Si-Ti coating, which had been applied by pack cementation at high temperatures over prolonged time periods.
The benefits of aluminizing an MCrAlX coating on superalloys for improved oxidation and corrosion resistance have been previously well documented. European Patent EP 897996, for example, describes the improvement of high temperature oxidation resistance of an MCrAlY on a superalloy by the application of an aluminide top coat using chemical vapour deposition techniques. Similarly, U.S. Pat. No. 3,874,901 describes a coating system for superalloys including the deposition of an aluminum overlay onto an MCrAlY using electron beam-physical vapour deposition to improve the hot corrosion and oxidation resistance of the coating by both enriching the near-surface of the MCrAlY with aluminum and by sealing structural defects in the overlay. Both of these systems relate to improvement of oxidation and/or hot corrosion resistance imparted to superalloys by the deposition of an MCrAlY thereon. These references do not relate to improvement of anticoking properties or corrosion resistance of high temperature stainless steel alloys used in the petrochemical industries. Such stainless steels have different chemical compositions and have higher levels of elements considered to be impurities. Examples of impurities include embedded nitrogen and carbon which diffuse outward when the alloys are heated and can shorten the life of improperly designed surface coatings.
A major difficulty in seeking an effective coating is the propensity of many applied coatings to fail to adhere to the tube alloy substrate under the specified high temperature operating conditions in hydrocarbon pyrolysis furnaces. Additionally, the coatings lack the necessary resistance to any or all of thermal stability, thermal shock, hot erosion, carburization, oxidation and sulfidation. A commercially viable product for olefins manufacture by hydrocarbon steam pyrolysis and for direct reduction of iron ores must be capable of providing the necessary coking and carburization resistance over an extended operating life while exhibiting thermal stability, hot erosion resistance and thermal shock resistance. It must also be capable of maintaining adherence over time as the impurities of the stainless steels diffuse outward.
Plasma transferred arc surface (PTAS), as disclosed for example in U.S. Pat. Nos. 4,878,953 and 5,624,717, is a technique used to apply coatings of different compositions and thickness onto conducting substrates. The material is fed in powder or wire form to a torch that generates an arc between a cathode and the work-piece. The arc generates plasma that heats up both the powder or wire and surface of the substrate, melting them and creating a liquid puddle, which on solidification creates a welded coating. By varying the feed rate of material, the speed of the torch, its distance to the substrate and the current that flows through the arc, it is possible to control thickness, microstructure, density and other properties of the coating (P. Harris and B. L. Smith, Metal Construction 15 (1983) 661-666). The technique has been used in several fields to prevent high temperature corrosion, including surfacing MCrAlYs on top of nickel based superalloys (G. A. Saltzman, P. Sahoo, Proc. IV National Thermal Spray Conference, 1991, pp 541-548), as well as surfacing high-chromium nickel based coatings on exhaust valves and other parts of internal combustion engines cylinders (Danish Patent 165,125, U.S. Pat. No. 5,958,332). PTA has not been used in applying MCrAlX coatings on stainless steel for purposes such as providing anti-coking and anti-hot corrosion on the inside of stainless steel tube and fittings used in ethylene pyrolysis furnaces.
MCrAlX alloys, where M=nickel, cobalt or iron or mixture thereof and X=yttrium, hafnium, zirconium, lanthanum or combination thereto and more specifically MrCrAlY alloys were discovered to be useful as coatings for the high temperature stainless steel tubes used in the petrochemical industry. When tubes used in ethylene furnaces were coated with this material, an improvement on the anti-coking, anti-carburization and resistance to hot erosion properties of the tubes were observed. The most successful process by which these coatings are deposited onto HTA tubes needs several steps: production of cathodes by plasma spraying of the powders onto a metallic tube substrate, transfer from the cathode to the tube""s inner surface by a sputtering process, and a heat treatment in the range of 1000 to 1160xc2x0 C. as disclosed in co-pending U.S. application Ser. No. 90/589,196. These operational steps suffer the loss of the raw materials used as active agents; in almost every step part of the material is lost, either due to an inherent partial transfer of material or by less than 100% yield. For some alloys it may be necessary to deposit an interlayer between the HTA substrate and MCrAlX alloy coating and then heat treat. The interlayer will then scatter nitrides and carbides that may precipitate inside the coating to avoid forming of an undesirable continuous layer during long term exposure to high temperatures in service. A continuous nitride or carbide layer would jeopardize the mechanical integrity of the films by reducing their adhesion to the tube.
These NiCrAlY anti-coking coatings generally need a special heat treatment to cause diffusion between the coating and the HTA tube. This heat treatment also serves the purpose of densifying and stabilizing the coatings. However, the hear treatment is an extra step requiring control of temperature, heating rate and dwell time to successfully produce a high quality coating.
Summary of the Invention
It is therefore a principal object of the present invention to provide a surface alloy on HTAs by a single process step without heat treatment to substantially eliminate or reduce the catalytic formation of coke on the internal surfaces of tubing, piping, fittings and other ancillary furnace hardware and to increase the carburization resistance thereof during ethylene production by pyrolysis of hydrocarbons or the direct reduction of oxide ores.
It is another object of the invention to provide a tightly-adherent McrAlX coating on HTAs which provides a some of aluminum for a protective alumina scale with few structural defects, thereby eliminating the need for a separate aluminizing step.
It is a further object of the invention, to provide a direct transfer of alloy coating material in powder or wire loan to the substrate to significantly cabs the efficiency of transfer with savings in material costs while intimately metallurigically bonding the coating to the HTA substrate.
Another important object of the invention is the provision of a denser, continuous, smooth interface between the alloy coating and the substrate with dispersed precipitated nitrides and carbides to obviate the need for a separate interlayer.
In its broad aspect, the method of the invention for providing a protective and inert coating to high temperature stainless steels comprises providing a protective and inert coating on high temperature stainless steel comprising metallurgically bonding a continuous coating of a MCrAlX alloy, where M=nickel, cobalt or iron or mixture thereof and X=yttrium, hafnium, zirconium, lanthanum or combination thereof, having about 10 to 40 wt % chromium, preferably about 10 to 25 wt % chromium, about 3 to 30 wt % aluminum, preferably about 4 to 20 wt % aluminum, and up to about 5 wt % X, preferably up to about 3 wt % X more preferably 0.25 to 1.5 wt % X, the balance M, by plasma transferred arc deposition of the coating onto a high temperature stainless steel substrate. The coating is deposited in a thickness of about 20 xcexcm to 6000 xcexcm, preferably 50 to 2000 xcexcm, more preferably 80 to 500 xcexcm onto the substrate.
The MCrAlX preferably is NiCrAlY and has, by weight about 12 to 25% chromium, about 4 to 15% aluminum and about 0.5 to 1.5% yttrium, the balance nickel.
In accordance with this preferred embodiment of the invention, the deposition of a dense, anti-coking NiCrAlY alloy coating art a single step on a HTA tube by plasma transferred arc deposition produces a gradual metallurgical bond between the alloy coating and substrate. The desired final thickness of the coating is between about 0.02 and 6 mm thick. The preferred thickness is in the range of 80 to 500 xcexcm in order to keep powder costs reasonable and to not unduly decrease the inner diameter of the tube.
The NiCrAlY alloy coating provides a source of aluminum to provide an xe2x88x9d=alumina based layer at the surface thereof by introducing an oxygen-containing gas such as air at a temperature above about 1000xc2x0 C. upon heating of the substrate and coating in a gaseous, oxidizing atmosphere such as air at a temperature above 1000xc2x0 C. in a separate step, or during commercial use by the introduction of or presence of an oxygen-containing gas at operating temperatures above about 1000xc2x0 C. The more complete the alumina scale the better the anticoking and anti-corrosion performance. Enhanced properties can be therefore sometimes be achieved by a further aluminizing step.
In accordance with another embodiment of the invention, however, the high temperature stainless steel substrate having a continuous coating of said MCrAlX alloy with a thickness of about 50 to 2000 xcexcm, preferably about 80 to 500 xcexcm, may be aluminized by depositing a layer of aluminum on the coating in a thickness up to about 50% of the coating thickness, preferably about 20% of the coating thickness, such as by thermal spray or magnetron sputtering physical vapour deposition. The system can be heated in an oxygen-containing atmosphere in a consecutive step or in a separate later step for a time effective to form a surface layer of xe2x88x9d=alumina thereon. Heat treating the coating with aluminum thereon and the substrate diffuses aluminum into the coating.